1. Field of the Invention
The present invention relates to a recording apparatus and a method of forming driving data for recording elements used in recording thereof, which are used in word processors, copy machines, facsimiles and so on, and used in a printer connected to a host computer for outputting information from host computer, and more particularly to a recording apparatus and a method of forming driving data used therefor using a serial type of recording head.
2. Description of the Prior Art
Characters and visualized images recorded on recording media such as recording sheets are digitized images generally formed by a set of pixels, each of which has individual density level (gray level) data. Each pixel is composed of a dot formed on the recording media by the recording head. As for recording heads for forming dots, a thermal transfer method and an ink jet recording method are widely known. Among them, an ink jet recording method has been widely used in recent years and it has many advantages in enabling relatively high fine-pitched dot formation and high speed recording.
In expressing gray scale of recorded images with several density levels, a dot density is assigned to each pixel according to these density level data, and the dot pattern for each pixel is determined in accordance with the dot density. Definition forms of the dot patterns are categorized into two groups, one refers to a method that a plurality of dots are placed in an identical position in responsive to the dot density, and the other refers to a method that a designated dot pattern is developed with a plurality of dots in response to the dot density.
Even though the dot pattern of each pixel has any patterns, it is necessary to determine driving data for driving recording elements of the recording head so as to form those dot patterns. In general, the driving data can be obtained as described hereinafter.
For example, performed are various processing such as .gamma. (Gamma)--correction, color correction, or undercolor eliminating processing as well as a multi-level halftoning (gray scale) processing (pseudo-halftoning process) corresponding to the number of dots which form each pixel. For this processing, such as an error diffusion method, a dither method, etc. are known in the art.
Referring to FIGS. 1 and 2, an example of conventional multi-level halftoning processing using the error diffusion method will be explained.
In FIG. 1, reference numeral 1001 denotes a specified pixel in which the multi-level halftoning processing is to be performed, and it has data I(x,y). Where I(x,y) is the product of original 8-bit density level data (representing levels between 0 to 255) of the specified pixel (x,y) and an error produced by the multi-level halftoning processing in the pixel in which the multi-level halftoning processing has been already performed. Numerals 1002 to 1008 denote pixels in which the multi-level halftoning processings have already been performed, and these pixels are surrounding pixels of the specified pixel. Each of constants a to g inside these pixels represents a multi-level value which is obtained by performing the multi-level halftoning processing. For example, in the case that a five-level halftoning processing is carried out, the five-level values a to g are integers among 0 to 4. FIG. 2 represents constants A to G for weighting the respective pixels in each of which the five-level halftoning processing has been performed. These constants are used to determine a threshold level for the five-level halftoning processing.
Firstly, mean density of the surrounding pixels is calculated by using the five-level data a to g of the pixels 1002 to 1008 which are the surrounding pixels of the specified pixel and in each of which the five-level halftoning possessing has already been performed, and weighted data A to G. More concretely, the mean density MV can be calculated by the following equation. EQU MV=a*A+b*B+c*C+d*D+e*E+f*F+g*G
Next, a threshold value for the five-level halftoning processing of the specified pixel is obtained by means of adding the mean density MV to each of predetermined threshold data stored in a memory, and then, this threshold value is compared with the density level data I (x,y) so that multiple-valued data P of the specified pixel is obtained as follows.
If MV+T&gt;I (x,y) then P=0. PA1 If MV+T.ltoreq.I (x,y)&lt;S1+MV+T, then P=1. PA1 If S1+MV+T.ltoreq.I (x,y)&lt;S2+MV+T, then P=2. PA1 If S2+MV+T.ltoreq.I (x,y)&lt;S3+MV+T, then P=3. PA1 If S3+MV+T.ltoreq.I (x, y), then P=4. PA1 [Where T=(S3-S2)/2=(S2-S1)/2] PA1 a recording head having a plurality of recording elements; PA1 a driving means for driving the plurality of recording elements of the recording head in accordance with driving data so as to form the dots; and PA1 an n-level halftoning processing means (n-1=the number of the driving data for forming the dots composing the pixel) for converting an input image data into the driving data of each of the pixels in such a manner that a two-level halftoning processing of the input image data is performed (n-1) times, and each result of the (n-1) times two-level halftoning processings is assigned as the driving data to one of the plurality of recording elements by each of the (n-1) times two-level halftoning processings. PA1 performing a two-level halftoning processing of an input image data, a result of the two-level halftoning processing being assigned as the driving data to one of the plurality of recording elements by the two-level halftoning processing; and PA1 repeating the two-level halftoning processing (n-1) times (n-1=the number of the driving data for forming the dots composing the pixel) so that n-level halftoning processing is performed and the input image data is converted into the driving data of each of the pixels.
Next, an error produced when the five-level halftoning processing of the density level data I(x,y) is performed, can be given as, EQU ERROR=p*S1+MV+T-I(x,y)
The ERROR is distributed at a certain predetermined rate to several pixels which are the surrounding pixels and in each of which the five-level halftoning processing has not yet been performed.
The five-level value of each pixel can be obtained by carrying out in order the processing described hereinabove on each pixel, wherein the five-level value corresponds to the number of dots of each pixel as mentioned above.
Known in the art are many kinds of methods of forming the plurality of dots which construct each pixel, on a recording media. For example, in an ink jet system, a multiple-droplet system is well known in the art, in which a single dot is formed in appearance by causing a plurality of ink-droplets to be projected substantially on an identical position of the recording media. Also known in the art is a recording method for separating one pixel into a plurality of segments and forming dots on each segment in correspondence with the density level data of the pixel.
However, both of them are constructed such that a plurality of dots of one pixel are formed by a single recording element. For example, in an ink jet system, the plurality of dots are formed by each of ink-droplets ejected from one ink orifice (ink jet nozzle). In such a system, when the ejected ink-droplets measure varies, the densities of pixels become different from each other. Thus, the deterioration in quality of picture arises in the case of multitude of different shades (lighter and darker variations) appeared in a recorded image. To solve such a problem, an improved system was proposed by the assignee of the present invention. In the improved system, one pixel is composed of a plurality of dots and each of these dots are formed by the respective different recording element. In case that the improved system and the conventional pseudo-halftoning processing are combined with each other, it is further necessary to assign each of recording elements to the multi-level value data of each pixel obtained by the pseudo-halftoning processing.